1. Field of the Invention
The present invention relates to an amplifying circuit for a transmitter, and more particularly to an amplifying circuit for a transmitter capable of reducing a noise.
2. Description of the Related Art
FIG. 5 is a view for explaining a conventional amplifying circuit for a transmitter, wherein FIG. 5A is a view for schematically explaining the amplifying circuit for a transmitter. In the figure, numeral 1 denotes an amplifier made up of a transistor connected by a push-pull circuit, numeral 2 donates an antenna that is a load of the amplifier and numeral 3 donates a driving circuit (driver) for driving the amplifier 1 that is provided with inversion circuits 31, 32 and 33 for inverting a driving signal supplied from a CPU 4 and supplying the resultant signal to the amplifier 1. Numeral 4 denotes the CPU for supplying a driving signal 41 to the driver 3. The CPU 4 modulates a carrier wave (carrier) with, for example, a control signal for locking a door of an automobile or a control signal for unlocking the same to thereby produce a PWM signal as a driving signal. The produced PWM signal is amplified by the amplifier 1 and then emitted to a subjective automobile.
FIG. 5B is a view showing an output waveform (view at the left side) from the driver 3 and a spectral distribution (view at the right side) of the driver output. As shown in the figure, the output from the driver has a PWM waveform having a predetermined cycle Tc that is modulated by, for example, a signal for unlocking the door.
FIG. 5C is a view showing an output waveform (view at the left side) from the amplifier 1 and a spectral distribution (view at the right side) of this output waveform. As shown in the figure, a rectangular wave is outputted in which a steepness of the waveform is reduced due to the intervention of the amplifier 1. Further, the spectral distribution at this time has a distribution in which a harmonic content is reduced. Specifically, it is understood that the harmonic content (2fc, 3fc, . . . ) of the carrier frequency fc corresponding to the above-mentioned predetermined cycle Tc is reduced.
FIG. 6 is a view for explaining in detail an amplifying circuit containing the amplifier 1. In this figure, numerals 11, 12, 13 and 14 denote switching devices each made up of an FET (Field Effect Transistor) or the like, these switching devices constituting the amplifier 1. The first switching device 11 and the second switching device 12 are connected in series between a power supply 10 and a ground E, while the third switching device 13 and the fourth switching device 14 are connected in series between the power supply 10 and the ground E. Further, the antenna 2 is connected between a junction point of the first switching device 11 and the second switching device 12 and a junction point of the third switching device 13 and the fourth switching device 14. Numerals 31, 32 and 33 each denotes an inversion circuit making up the driving circuit.
Numeral 41 denotes a driving signal of the first switching device 11, this signal being supplied to a control electrode of the first switching device 11. Numerals 42, 43 and 44 each denote a driving signal of the second switching device 12, third switching device 13 and the fourth switching device 14 respectively, each signal being supplied to a control electrode of each of the second switching device 12, the third switching device 13 and the fourth switching device 14.
As described above, the CPU 4 modulates the carrier wave (carrier) with, for example, the control signal for locking the door of an automobile or the control signal for unlocking the same to thereby produce the PWM signal as the driving signal.
At first, when the driving signal 41 produced by the CPU 4 is in H-level, this H-level driving signal 41 energizes the first switching device 11 to turn it on. At this time, the driving signal 41 is inverted by the inversion circuit 32 to become the L-level driving signal 42 that then energizes the second switching device 12 to turn it off. When the driving signal 41 produced by the CPU 4 is in L-level, this L-level driving signal energizes the first switching device 11 to turn it off. At this time, the driving signal 41 is inverted by the inversion circuit 32 to become the H-level driving signal that then energizes the second switching device 12 to turn it on.
On the other hand, the driving signal 41 produced by the CPU 4 is supplied to the third switching device 13 through the inversion circuit 31, and further, to the fourth switching device 14 through the inversion circuit 33.
In this case, when the driving signal 43 that inverts the driving signal 41 produced by the CPU 4 is in L-level, this L-level driving signal 43 energizes the third switching device 13 to turn it off. At this time, the driving signal 43 is inverted by the inversion circuit 33 to become an H-level driving signal 44. This H-level signal energizes the fourth switching device 14 to turn it on. When the driving signal 43 that inverts the driving signal 41 produced by the CPU 4 is in H-level, this H-level driving signal 43 energizes the third switching device 13 to turn it on. At this time, the driving signal 43 is inverted by the inversion circuit 33 to become the L-level driving signal 44 that then energizes the fourth switching device 14 to turn it off.
Specifically, when the driving signal 41 produced by the CPU 4 is in H-level, the first switching device 11 and the fourth switching device 14 are turned on, while the second switching device 12 and the third switching device 13 are turned off. When the driving signal 41 produced by the CPU 4 is in L-level, the first switching device 11 and the fourth switching device 14 are turned off, while the second switching device 12 and the third switching device 13 are turned on.
In the case where the driving signal 41 is transmitted through the inversion circuits 31, 32 and 33, a delay occurs in the signal transmission. When the delay occurs in the signal transmission, the first switching device 11 is turned on before the second switching device 12 is turned off, for example, meaning that the first switching device 11 and the second switching device 12 are simultaneously energized. Similarly, in the case where the third switching device 13 is turned on before the fourth switching device 14 is turned off, the third switching device 13 and the fourth switching device 14 are simultaneously energized.
When the first switching device 11 and the second switching device 12 or the third switching device 13 and the fourth switching device 14 are simultaneously energized, feedthrough current flows through these switching devices. This feedthrough current is peak current having a great peak value and a steep rising, by which the voltage of the power supply 10 greatly fluctuates. This peak current or the fluctuation in the power supply voltage is transmitted to the antenna 2 to thereby be emitted to the outside as noise.
FIG. 7 is a view for explaining a conventional method for controlling a noise. In the figure, numerals 15 and 16 denote resistances for controlling the feedthrough current, wherein the resistance 15 is connected between the first switching device 11 and the power supply 10, while the resistance 16 is connected between the third switching device 13 and the power supply 10. The resistances for controlling the feedthrough current control the peak value or the steepness at the rising of the feedthrough current, so that the noise caused by the peak value or the steepness at the rising of the feedthrough current can be controlled.
When, for example, the first switching device 11 and the second switching device 12 are simultaneously energized as described above in the amplifier made up of the push-pull-connected transistor, not only the noise occurs but also the efficiency of the amplifier 1 reduces and further, there may be the case where the switching devices are destroyed due to the above-mentioned feedthrough current. Further, in the case where the resistances for controlling the feedthrough current for controlling the feedthrough current are used as described above, a countermeasure is required for radiating heat from these resistances.